1. Field of the Invention
The present invention relates to a wheel for vehicles. In particular, the present invention relates to wheels provided with devices for generating electric energy needed for powering devices installed in the tyre or on the rim of said wheel.
2. Description Of Related Art
These devices are, for instance, defined by detecting units capable of monitoring the operating conditions (pressure and temperature, for example) of the tyres and possibly adapted to keep a trace of the evolution in time of parameters representative of this operation, or they can be embodied by actuators such as pumps adapted to keep pressure within the tyre substantially constant.
The parameters that are generally considered may be identification code, temperature, pressure, distance covered by the tyre, load, as well as parameters resulting from mathematical processing operations that can be executed within the tyre itself (such as forces, accelerations, deformations).
In particular, in case of vehicles using tyres of the run flat type, i.e. tyres capable of ensuring that some kilometric distance may be run even with a deflated tyre, provided some operating parameters are observed such as maximum speed, temperature and maximum distance that can be travelled over, said detecting units become of fundamental importance to enable safe use of said type of tyres.
For the purpose, a detecting system can be mounted inside the tyre which may comprise at least one detecting unit, a control and/or data storage unit (such as a microprocessor), and an antenna. The antenna's task is to enable radio-frequency signal exchange with the devices mounted on board the vehicle. In addition, the detecting system comprises a powering unit adapted to supply the electric energy required for operation of same.
Document WO 2006/048899, in the name of the same Applicant, relates to a tyre for vehicle wheels of the type provided with an antenna to enable transmission and/or reception of operating parameters of the tyre itself, for example. The antenna is also used to feed the detecting unit and the devices associated therewith, so as to avoid use of self-governing powering units placed within the tyre. For the purpose the antenna has a coil-shaped conformation so as to enable coupling with an electromagnetic field generated by a device present on board the vehicle, and to absorb the necessary energy for feeding the detecting unit.
Document WO 2005/067073, in the name of the same Applicant, shows a tyre comprising a flexible piezoelectric element associated with a device for energy storage. The flexible piezoelectric element is mounted in overhanging in a container so that it can be positioned substantially along a plane perpendicular to a radial direction of the tyre and in such a manner that a first end of the piezoelectric element is linked to the container. A mass is engaged with the second end of the flexible piezoelectric element. A small interspace is defined between the inner walls of the container and the outer surface of the mass, to enable a limited bending of the piezoelectric element. The container holding the piezoelectric element is mounted to a portion of the tyre close to the tread, preferably on an inner surface of the tyre. The piezoelectric element bends under the radial-acceleration action when the tyre rolls.
U.S. Pat. No. 5,941,692 shows a pump provided with a resonant oscillating mass suitably adjusted, which pump is installed within the tyre or on the wheel rims of motor-vehicles, to maintain the optimal tyre pressure and generate electric energy through movement of said oscillating mass. This oscillating mass can be a pendulum or a ball free to roll in an arched chamber. The oscillating mass takes its energy from the gravitational field for rotation of the tyre/rim on which the mass itself is installed. This mass oscillates under the influence of the force of gravity and the centripetal radial force. Oscillation of the pendulum/ball can be employed to deflect a piezoelectric element whose output electric signal can be used to energise sensors or other electric apparatus. The electric energy can also be generated by movement of the pendulum/ball, through magnetisation of the oscillating mass and making this mass pass close to a rotor/coil.
In this technical field, the Applicant has felt the necessity:                to improve the construction simplicity of the devices for generating electric energy manufactured according to the teachings of the above mentioned documents;        to increase reliability of said devices;        to reduce bulkiness and weight of same;        to increase the efficiency of these devices as compared with known devices, in order to at most exploit the energy produced by the tyre deformation and/or movement of the wheel;        to enable a quick and easy installation, without requiring any part of the device being installed out of the wheel.        
The Applicant has noticed that, since as regards the antenna and the device for generating the magnetic field described in document WO 2006/048899 mounting of elements on the vehicle is required, installation of same needs modification of the vehicle itself and is therefore complicated. In addition, each time replacement of the tyre is required, it is of the greatest importance that the new tyre should contain elements (antenna, detecting unit, etc.) that are compatible with the device installed on the vehicle.
The Applicant has further found that the powering unit of document WO 2005/067073 is not able to fully exploit the kinetic energy generated by rolling of the tyre. In fact, deformation of the piezoelectric element, due to the forces acting on the mass, is necessarily limited by the presence of the container's inner wall performing the function of stop limit, under pain of breakage of the piezoelectric element itself. Therefore, forces of inertia that, in the absence of said stop limit, would cause a greater deformation of the piezoelectric element cannot be fully converted into electric energy.
The Applicant has also observed that the resonant oscillating mass described in document U.S. Pat. No. 5,941,692 must necessarily be installed on one side of the wheel with the oscillation axis parallel to the rotation axis of the wheel itself, so that the centripetal force and the force of gravity may cause oscillation of same, and this constraint is a limitation in terms of flexibility of use.
The Applicant has finally noticed that the embodiments shown in documents WO 2005/067073 and U.S. Pat. No. 5,941,692 hardly succeed in being very efficient in converting the mechanical energy into electric energy, said mechanical energy being generated by the forces acting on the tyre, due to the constraints provided in these embodiments, all having a stop limit.